This FIRCA supplement will complement the research program of D.M. Dacey entitled, "Neural Organization of the Primate Inner Retina" (R0IEY06678). That project focuses on the physiology and anatomy of the primate inner retina including the ganglion cells and their connections with bipolar cells. The proposed extension of the project focuses on the outer retina, with a long-term objective to understand the structure and circuitry of the horizontal cells and their functional role in parallel chromatic and achromatic signal processing. Little is currently known about the physiology of horizontal cells of the macaque retina in general, and specifically about their responses to color. To test the hypothesis that the macaque retina contains both luminosity (L-) and chromatic (C-) types of horizontal cells the following specific aims are proposed: 1) To define the spectral responses of the horizontal cells using equiquantum monochromatic stimuli and intracellular recording, and, thus to determine the spectral type (L-type or C-type) of the cells; 2) To examine the spatial organization of receptive fields of horizontal cells and their spectral sensitivity using monochromatic color stimuli of different intensities, forms (spot and annular), and sizes. 3) To identify individual horizontal cells morphologically using dyes injected into the neurons under study. Correlating the anatomy and physiology for the major primate horizontal cell types will extend the understanding of the neural basis for chromatic and achromatic visual processing from the inner retina (focus of the parent grant) into the outer retina (focus of the proposed supplement). This research will contribute to understanding the neural basis of diseases, such as glaucoma, that affect specific cell types in the retina.